seconds; some surface and all of the bound moisture
are removed in the fluid-bed stage by holding the
product at suitable drying temperatures for about 30
min.
Usually, wet cake with 22 to 25% water (wb) is fed
to the dryer by a screw conveyor and enters by a
special mill that deagglomerates the feed material,
disperses it into the drying airstream, and accelerates
it to duct velocity. The mill should handle the feed
gently, as PVC is sensitive to high shear.
The flash dryer stage discharges the product to the
fluid bed between 2 and 8% water, with the intermedi-
ate moisture chosen according to the basis of opti-
mization used. Flash dryer air temperature may be
typically 180
8
C at the inlet and 60
8
C at the outlet,
depending on moisture content and the drying char-
acteristics of the particular resin. As already men-
tioned, S-PVC is sensitive to shear; for this reason,
dry duct velocities are kept low (around 15 m/s) and
care is exercised in handling the dried product.
It is possible to arrive at the required product final
moisture content by flash drying alone but, because of
the residence time available in the flash dryer, the high
temperatures required give an unsatisfactory product.
Moreover, there is a very wide range of S-PVC homo-
polymers, varying in molecular weight, particle size,
and other properties, and all have different dewater-
ing and drying characteristics.
The benefits achieved in a two-stage system are its
ability to handle upsets in inlet moisture in the flash
dryer, a lower energy cost, and a relatively simple
scale-up.
Modest improvements with respect to the most
economical drying of S-PVC are a continuous, sin-
gle-stage, contact fluidized bed dryer, as shown in
Figure 41.9. In this type of dryer, the concepts of
back-mixed fluidization and plug-flow fluidization
are advantageously combined in a single unit. A
broad residence time distribution is obtained in a
back-mixed fluid bed in which the bed itself has a
relatively small length/width ratio. In performance it
can be compared with an agitated tank provided
with overflow, inasmuch as the vigorous mixing in-
side the fluid bed will result in a uniform temperature
and constant average moisture content of the par-
ticles throughout the entire bed. The product dis-
charged from this back-mixed fluid bed has the
same temperature and moisture content as the bulk
material inside the fluid bed. Further, because of the
excellent heat and mass transfer between the fluid-
ized particles and the drying air, equilibrium is
reached between the exhaust air and the product
inside the bed. This type of fluid-bed drying concept
is found to be very suitable for drying surface mois-
ture when residence time has no impact on the dry-
ing performance.
After the mixed-bed section, a plug-flow section is
provided in which the final drying of PVC takes place.
This section is fairly small compared with the back-
mixed section and is usually obtained by dividing the
fluid bed into compartments. This concept is particu-
larly advantageous for drying bound moisture from
heat-sensitive materials since the residence time is
controlled within the narrow limits and a distinct
Contact
fluid bed
(back-mixed part)
Plug
flow
Cyclone
Heater
Air filter
Product
FIGURE 41.9
Contact fluidizer for suspension-grade polyvinyl chloride.
ß
2006 by Taylor & Francis Group, LLC.
moisture profile can be obtained along the length of
the unit because of a very low degree of back-mixing.
In this type of drying system for S-PVC, wet PVC
cake is usually transported from the decanter centri-
fuge by a screw feeder to the product distributor of the
back-mixed fluid-bed section. It then flows through an
overflow weir into the plug-flow section where the final
drying takes place. Finally, the product is discharged
through the discharge weir arrangement.
The back-mixed section of the unit is provided
with heating panels; no heating panels are provided
in the plug-flow section, partly because the cost can-
not be justified and partly because of the tendency for
electrostatic deposits on the heating panel encoun-
tered with PVC at low moisture content to decrease
the heat transfer coefficient.
The contact fluidized bed provided with heating
panels appears to have proven to be superior to the
flash fluid-bed drying system from the point of view
of heat economy and overall savings. The contact
fluidizer does have a few limitations. First, it is man-
datory that the polymer material be readily fluidizable
at a moisture level well above the moisture level in the
back-mixed section to avoid defluidization of the bed
during upset conditions. Second, the centrifuge cake
should not be too sticky and have too much tendency
to form agglomerates of the individual polymer par-
ticles. In such a case, a flash dryer is better suited as
the predrying stage as better disintegration takes
place in the venturi section of a flash dryer than in a
back-mixed fluid bed.
Although a fluid bed as a second-stage dryer gives
accurate product temperature control while providing
adequate residence time, depending on the predryer
load, evaporative load in this stage may be small.
This results in a low airflow requirement and makes
fluidization more difficult. In such cases, a vibrat-
ing fluid-bed design is a better alternative. Here,
PVC is conveyed by vibration, permitting varying
gas speeds without affecting the conveying rate or
residence time. Also, with the low airflow rates of
the vibrating fluid bed, the fines pickup problem (nor-
mally associated with high gas flow rates) is minim-
ized and, as the vibration is at a low frequency, the
overall effect of the gas and vibration is to transport
the product gently, minimizing damage. The vibrat-
ing FBD must be among the most important but
underutilized dryer of all granular products.
During fluidization of PVC, electrostatic charges
arise of such magnitude that they affect the hydro-
dynamics of the system. This is disadvantageous for
transfer processes in the bed, e.g., for heat transfer
between the heating surface and the bed. This is a
difficult problem in a fluidized bed because of inten-
sive movement of particles and frequent interparticle
and particle–wall contact. Although charge gener-
ation cannot be prevented, one can limit its magni-
tude (and try to increase its dissipation) by changing
process conditions. One method is the addition of a
small portion of fines to the bulk; this results in the
splitting of agglomerates and disappearance of the
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